1. Field of Invention
This invention relates to a method of making contacts in integrated circuits. More particularly the invention relates to a method of making low resistance contacts between levels of metallization in integrated semiconductor circuits, where the contact openings and channels are cleaned by means of cathode sputtering. The invention also comprises a device for carrying out said method.
2. Background Art
If aluminum metallization is used in semiconductor technology, making good contact connections from one level of metallization to the next one is a problem because when aluminum or an aluminum alloy with a high oxygen affinity such as aluminum-copper is exposed to air, a strongly adhering, thin oxide skin forms thereon because of the high oxygen affinity of aluminum. In circuits with low circuit density and relatively few but large contact openings this problem is solved in an uncomplicated manner in that prior to the application of the next level of metallization the semiconductor substrates are etched with dilute buffered hydrofluoric acid. As a consequence, a thin, noncontiguous aluminum oxide layer is formed in the contact holes. A heat processing of the thus prepared substrates following metallization at approximately 400.degree. C. causes an aluminum self-diffusion and recrystallization at the original metal to metal interface in the contact openings. In this manner, useful and low resistance contacts are obtained.
It has been suggested (IBM-TDB Vol. 19, No. 1, 1976, p. 20) to provide a zinc coating on the previously precleaned metal surface by the immersion of wafers in a zinc sulfate-hydrofluoric acid. The wafers are then heated to approximately 200.degree. to 250.degree. C. and the respective metal is deposited. Following the metal deposition, the wafers are heated in an inert atmosphere for approximately 30 minutes to approximately 350.degree. C., with the consequence of the out-diffusion of zinc atoms into adjacent metal layers, and therefore of low resistance contacts.
It has also been suggested to use an intermediate layer of titanium between the metallization levels. The titanium film reduces the aluminum oxide in the area of the via hole and thus effects a reduction of the contact resistance between the two conductor planes. The disadvantage of this method is that it is difficult to control, and that the reduction of the contact resistance achieved thereby is insufficient in connection with small via holes (d&lt;5 .mu.m).
In high density integrated circuits where the diameter of the holes decrease and the number of holes increase drastically, conventional cleaning methods can no longer be used. It has therefore become necessary to find an effective cleaning process for via holes. An in situ cleaning process by means of cathode sputtering was developed (IBM-TDB Vol. 20, No. 2, 1977, pp. 574 to 576). Prior to the evaporation of the second metallization level the surface of the first metallization level is cleaned according to this process by means of cathode sputtering, i.e. by applying a high frequency voltage to the wafer holder in an argon atmosphere at low pressure. The conditions for cathode sputtering were such that approximately 10 nm aluminum or aluminum oxide are removed. With this process, the contact resistance is practically reduced to zero. The effectivity of the method is particularly evident when wafers which had been cleaned in a vacuum system by means of cathode sputtering, and subsequently exposed to air for several minutes prior to the vapor deposition of the next metallization level, are compared with wafers which after cleaning by means of cathode sputtering are applied with a metallization level by vapor deposition without the vacuum having been interrupted. It can be demonstrated that under the former conditions aluminum again oxidizes quickly, forming an oxide barrier with a layer thickness of approximately 1.5 to 2.0 nm. The in situ cleaning process by means of cathode sputtering without an interruption of the vacuum is therefore particularly advantageous for high product yields.
However, this method cannot be effectively applied in processes using masks of temperature-sensitive photoresist materials. The reason for this is the length of time required for effective sputter cleaning. During sputter cleaning, the resist reticulates and partly polymerizes due to the argon ion bombardment and high temperature associated with prolonged bombardment and the resist can no longer be removed from the substrate after this process. Prolonged ion bombardment is required to obtain a clean, oxide-free contact in a via because the level of water vapor in the chamber atmosphere causes reoxidation to occur on the previously cleaned substrate surface, thus requiring repeated sputtering until reoxidation does not occur. For example, bare aluminum contacts which are cleaned react with water vapor, and form an oxide layer on the aluminum. This oxide layer is subsequently removed again in the sputtering process. A clean, oxide-free surface is maintained on the aluminum coated wafer holder and in the via when the sputtering reaches a point in time where all residual water vapor has been consumed in the reaction with the aluminum metal.
Thus, to avoid prolonged ion bombardment, and still obtain a clean, oxide-free metal to metal contact in via connections, a low residual water vapor is required in the vacuum work chamber where the wafers are located. Vacuum conditions in standard high vacuum chambers, (i.e. standard chambers are built with seals and gaskets made of rubber material such as viton), are insufficient to maintain an oxide free contact area after the removal of an oxide layer from a contact by sputter cleaning and before the next layer of metal is deposited by evaporation. Standard high vacuum tooling is required in manufacturing type equipment because the manufacturing chambers must be continually and rapidly accessed to move parts in and out thereof.
Considering the restraint that the chamber be of the standard high vacuum type for manufacturing purposes, the problem is how to eliminate oxygen, and oxygen containing compounds, particularly water vapor in the work chamber, which causes reoxidation after sputter cleaning. A Meissner trap may commonly be used to lower the residual water vapor in a vacuum chamber. However, Meissner traps do not reduce the level of residual water vapor low enough to prevent re-oxidation of the sputter cleaned contacts.
Commercial ion getter pumps are also available to reduce the residual water vapor in vacuum work chambers. There are basically two types of ion getter pumps, one operates by evaporation mechanisms and the other by sputtering. These getter pumps are designed to be attached to a flange or a port in a vacuum work chamber. However, these ion getter pumps require ultra high vacuum work chambers to operate effectively, and are thus not suited for use with standard vacuum chambers as are required for manufacturing. Ultra high vacuum chambers require metal gaskets and fastening means such as large, heavy bolts. Such ultra-high vacuum tooling could not be effectively used in a manufacturing mode wherein substrates are continually entered into and removed from the chamber.
Standard high vacuum tooling, which is compatible with rapid access use as required in the manufacturing mode, is not suited for use with commercial ion getter pumps. One reason for this is that vacuum chambers that are pumped with ion getter pumps must be able to withstand relatively high temperatures to drive residual gases from the chamber walls, e.g. temperatures in the range of 400.degree. C. If the chamber walls were not heated, ion getter pumps would not be very effective because of the high outgassing rate of residual gases from the walls of the chamber into the chamber itself. If the chamber walls were heated, ion time if used with standard high vacuum chamber tooling getting pumps would lose their effectiveness in a short because the high temperatures required to drive the residual gases from the chamber walls would cause the rubber seals to leak. When using an ion getter pump, there is a relatively low pressure conductance between the work chamber and getter pump, (i.e. low pressure conductance exists because of pressure drops in the connections and shielding baffles which keep getter material from getting into the work chamber). Due to this low conductance, the pump is effective in its own chamber, but not very effective in the work chamber where residual water vapor would exist for extended periods causing reoxidation on metal surfaces after cleaning.
An additional problem with using ion getter pumps in conjunction with a work chamber to remove water vapor therefrom is that these pumps are generally not selective in the gases that they remove from the chamber. These pumps remove all species of residual gases in the chamber. The only gases that are of concern for the present application are those containing oxygen, particularly water vapor. Because the commercially available ion gettering equipment pumps all gases, they incorporate means for generation of a magnetic field to enhance pumping action. This makes such equipment bulky and awkward to use and does not allow one to place such pumps in close proximity to the wafers or workpieces inside the vacuum work chamber.
It is therefore a primary object of the invention to provide a method for cleaning semiconductor substrates covered with masks of temperature-sensitive resist materials, by means of a more effective cathode sputtering method wherein shorter sputter clean cycles can be used.
It is another object to provide a method for cleaning semiconductor substrates by cathode sputtering in a manufacturing mode, wherein rapid access is required to a standard high vacuum work chamber, and ultra high vacuum tooling is not required.
It is another object to selectively remove water vapor or other oxygen containing gases from the work chamber, particularly in the area of the chamber in close proximity to the wafers.
It is another object to provide an effective means for carrying out this method and for improving the vacuum conditions in a sputtering deposition chamber.